An I.C. (integrated circuit) device is formed by an I.C. chip encapsulated within a resin package, as by a molding operation, and the chip connects to a plurality of pins (generally referred to as "leads") which project outwardly beyond the sides of the resin package. These leads are part of a lead frame which is generally elongated so that several such I.C. devices are formed longitudinally in spaced relationship therealong, with the I.C. devices thereafter being separated or severed from the longitudinally extending edge strips of the lead frame.
As is well known, the leads are typically coated with solder, which coating generally occurs in one of two different ways. In a first common technique, each I.C. device is separated from the lead frame, its leads are then bent downwardly substantially at right angles at their point of projection from the resin package, and the bent leads of the individual I.C. device are then dipped into a bath of liquid solder. The second conventional technique involves maintaining the plurality of I.C. devices integrally connected to the elongate lead frame so as to define a relatively flat package having a plurality of I.C. devices thereon, with this package in its entirety then being dipped into a bath of liquid solder.
The above commonly utilized soldering techniques, however, are known to possess several disadvantages.
For example, with respect to the first technique mentioned above, this is disadvantageous since the number of processing and handling steps are significant, and this process is thus both complex and inefficient.
A further disadvantage associated with these techniques is the inability to coat the leads with a film having the desired uniformity of thickness. These conventional techniques often result in extremely heavy or thick solder films on the leads, and particularly with I.C. devices wherein the width between the leads is extremely small, these soldering techniques often tend to create solder bridges which extend between the leads due to the surface tension of the solder. The formation of such bridges causes the I.C. device to be defective.
Still a further disadvantage associated particularly with the second technique is due to the fact that the entire I.C. device is dipped in the hot solder bath. Since the melting point of commonly used solder material is about 200.degree. C., and since the molten material is maintained at least at about this temperature, which temperature will exceed the typical temperature of the lead frame and I.C. devices by about 170.degree. C., this excessive temperature difference imposes a substantial heat shock on the I.C. devices when they are immersed into the solder bath. This heat shock can cause significant damage, such as to the I.C. chip within the resin package. Thus, use of this technique has been observed to significantly increase the number of defects and hence the rejection rate of the I.C. devices.
It is thus an object of this invention to provide a soldering apparatus, particularly for soldering the leads associated with I.C. devices mounted on a lead frame, which apparatus successfully overcomes or significantly minimizes many of the known disadvantages as associated with conventional techniques of the type described above.
More specifically, the present invention relates to an apparatus which permits automatic and efficient solder coating of the leads associated with a plurality of I.C. devices as mounted on a lead frame prior to severing of the I.C. devices from the lead frame. This apparatus in particular provides for application of a desired uniform thickness of coating on the leads, while at the same time substantially prevents creation of solder bridges between the leads. This apparatus also prevents the resin packages from contacting the hot molten solder, and thus avoids severe heat shock to the I.C. devices. The rejection rate of I.C. devices is hence significantly reduced, whereby I.C. devices can be solder coated in a more efficient and economical manner.
In the apparatus of this invention, a pair of rollers assemblies are provided for pinching the lead sections of the lead frame therebetween and applying a thin coating or film of solder thereto. The pair of roller assemblies each includes two axially spaced rollers which respectively cooperate with similar axially spaced rollers associated with the other assembly, thereby defining two cooperating sets of counter-rotating rollers which are axially spaced apart. At least one of tee rollers has a lower part thereof which passes through a molten solder bath. The lead frame is passed between the rollers so that the leads associated with opposite sides of the resin package are effectively pinched or passed through the nip defined between the opposed rollers of the two sets, whereby a thin solder film is applied to at least one side of the leads. At the same time the enlarged resin package passes freely through the space defined between the axially spaced rollers so as to be free of any significant contact with the hot molten solder.
In the apparatus of this invention, the pair of axially spaced rollers associated with each assembly are preferably capable of being shifted axially relative to their rotational axis to increase or decrease the axial spacing therebetween to accommodate the size of the resin package which passes through said space.
Other objects and purposes of the invention will be apparent upon reading the following specification and inspecting the accompanying drawings.